The invention relates to a screen for cleaning a fiber suspension.
Screens are machines used in the paper industry to clean a pulp suspension comprising water, fibers, and dirt particles. Here a feed flow runs through a screening device, where the accept flow, consisting of water and fibers, flows through the screen. A partial flow, known as the reject and consisting of water, fibers, and dirt particles, is generally removed at the opposite end to the feed flow. Thus, the solids particles present in the liquid are separated from one another in the screens. By contrast, in filtration processes the liquid is separated from the solids.
In general, a screen of this type is rotationally symmetrical and consists of a housing with a feed device mounted at a tangent, a cylindrical screen basket, normally with perforations or vertical slots, and a rotating rotor. The purpose of the rotor is to keep the screen slots clear, achieved by the vanes rotating close to the screen surface. The accept is collected in a so-called accept chamber, which often has a conical design, and drawn off from here in radial direction. The reject flow is generally brought to a reject chamber, which is usually annular, located at the opposite side of the screen basket to the inlet, and drawn off from here at a tangent.
A screen of this type is known, for example from U.S. Pat. No. 4,268,381.
Other screens known are described in, for example, EP 1 122 358 A2, EP 1 124 002 A2, and EP 1 124 003 A2.
In the screens according to EP 1 122 358 A2, EP 1 124 002 A2, and EP 1 124 003 A2, the following measures are implemented, particularly in order to improve flow conditions:                An additional screen basket is provided in the feed area for pre-screening.        In the feed area between the pipe socket and the freely accessible end of the rotor there is a stationary mounting, particularly a cone, truncated cone, hemisphere, spherical segment, spherical segment between two parallel circles, paraboloid, or a hyperboloid of two sheets.        The accept chamber is designed as twin cones, widening in flow direction of the pulp suspension and tapering again from the mouth of the accept outlet in a conical shape towards the reject outlet.        
In these known screens the rotor is designed for even flow onto the screen and is parabolic in shape so that the axial flow speed inside the screen basket remains constant at an assumed uniform flow through the screen basket. As an alternative, a cone shape can be used to come closer to the parabolic shape of the rotor.
It is also known that screens can be designed as multi-stage units, comprising several separation stages one after another.
The screens known from the state of the art, however, still hold disadvantages. In particular, the flow conditions at the reject outlet leave much to be desired.